Thermal barriers for electric cables



Sept. 26, 1967 P. c. WOODLAND ETAL 3,344,228

THERMAL BARRIERS FOR ELECTRIC CABLES Filed Nov. 19, 1964 Adhes/ve Polymer under fesf, C

' v Zmf/ 00% es/ s e :1 E 50 m f/ a/um in um 207 7007785176 5/: ee 2; H

W 5 smkj 'INVENTORS Pou/ C. Wood/0nd Gera/dEC/ock 6 Raymond C. Mi/oner' Y 9; g 5 g HTTORNEYS United States Patent Delaware Filed Nov. 19, 1964, Ser. No. 412,445 18 Claims. (Cl. 174-107) This invention relates to improved electric cables and more particularly it relates to electric cables having 1mproved thermal insulation barriers.

Various materials are being employed as thermal insu lating barriers to protect cable cores from heat damage during cable fabrication processes. For example, plastic tapes, rubber-plastic laminates and the like are employed as thermal barriers between a metallic shield and an inner conductor core as in communication cables. The thermal .barriers are employed to protect the inner conductor core from heat damage such as from the heat of extrusion where a polymer jacket is extruded over the metallic shield or from an external source of heat employed to heat the metallic shield to promote the adhesion of a polymer jacket to the metallic shield. However, the above described thermal barriers are not entirely satisfactory because they are expensive, require additional handling to apply the barrier to the cable and also because they have undesirably high dielectric constants.

The present invention provides an improved electric cable wherein a cable core having at least one insulated metal conductor is surrounded by a layer of a foamed polymer composition. The layer of foamed polymer is surrounded by a supporting sheathing member for said foamed layer and the supporting sheathing member is surrounded by a layer of a thermoplastic polymer.

FIGURES 1 through 4 of the accompanying drawing are schematic end views of cable constructions employed in practicing the present invention. FIGURE 5 is a schematic side view of an assembly of parts employed to test the thermal insulating property of various thermally insulating materials.

FIGURE 1 represents one embodiment of the present invention, wherein a cable core 1 having at least one insulated metal conductor is surrounded by a layer of a foamed insulating composition 2. The foam layer 2 is surrounded by and attached to a metal shield 3 and the metal shield is surrounded by an outer polymer jacket 4.

FIGURE 2 represents another embodiment of the invention wherein a cable core 12 having at least one insulated metal conductor is surrounded by a layer of a foamed thermally insulating composition 13. The foamed layer 13 is surrounded by and attached to a metal shield 1-4, said shield being coated with a thin layer of adhesive polymer 15. An outer polymer jacket 16 surrounds the adhesive-coated metal shield 14.

FIGURE 3 represents another embodiment of the invention, wherein a cable core 21 having at least one insulated metal conductor is surrounded by a layer of a foamed, thermally insulating composition 22. The foamed layer 22 is surrounded by a metal shield 24, said shield being coated on both sides with a thin layer of adhesive polymer i.e. layers 23 and 25. An outer polymer jacket 26 surrounds the adhesive-coated metal shield 24.

FIGURE 4 represents still another embodiment of the invention wherein a cable core 32 having at least one insulated metal conductor is surrounded by a layer of a foamed thermally insulating composition 33. The foamed layer 33 is surrounded by and attached to a thin layer of a plastic material 34. An outer polymer jacket 35 surrounds the thin layer of plastic material 34. In a variation of the above-described cable assembly, the foamed 3,344,228 Patented Sept. 26, 1967 "ice layer 33 is positioned between the plastic layer 34 and the outer polymer jacket 35.

Cables in accordance with the present invention can be constructed by (1) coating a thin metal or plastic substrate on at least one side with a layer of an adhesive polymer, (2) coating one side of the metal or plastic substrate with a thin layer of a foamable coating composition, i.e. a composition which, on being activated by heating, expands to form a lower density, cellular structure, (3) wrapping or folding the resulting laminate around the cable core to form a longitudinal sheath or shield, the foamable coating composition being adjacent the cable core and (4) extruding a heat-plastified layer of a plastic material over the metal or plastic substrates. The heat of extrusion causes the foamable coating composition to expand to form a layer of cellular polymer between the metal or plastic substrate and cable core. The heat of extrusion also causes those layers having adhesive polymer between them to become firmly bonded together. It is desired that temperatures of from about C. to about C. be attained to promote adequate bonding between the layers. It is to be understood that a metal or plastic substrate can be coated with a foamable coating composition as described in -(2) above and subjected to heat to expand the polymer composition before assembling the cable as described above.

The foamable composition employed in the present invention preferably contains (1) fine, discrete polymer particles, each particle containing a volatile organic blowing agent and (2) a binder. The particulate polymer particles containing blowing agent can be produced in accordance with the methods as described in U. S. Patents 2,681,321, 2,744,291 and 2,862,834 and preferably by the method of Belgian Patent 641,711. The expansible polymer particles are mixed with a binder, such as a latex,

to form a liquid foamable coating composition.

The adhesive composition that can be employed to coat either side of the metal shield before applying the foamable coating composition or the outer polymer jacket may be of any composition which adhesively bonds the resulting foamed polymer layer or outer polymer jacket to the metal shield. A preferred adhesive polymer comprises a random or graft copolymer of a major proportion of ethylene and from about 3 to about 20 percent by weight based on the copolymer of an acidic copolymer of an acidic comonomer selected from a,,8-ethylenically unsaturated monoand polycarboxylic acids and acid anhydrides having from 3 to 8 carbon atoms per molecule and partial esters of such polycarboxylic acids wherein the acid mole ty has at least one carboxyl group and the alcohol moiety has from 1 to 20 carbon atoms. Specific examples of such acidic comonomers are acrylic acid, methacrylic acid, crotonic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, monomethyl maleate, monomethyl furnarate, and monoethyl fumarate.

It has been found that the particular foamed thermal barrier of the present invention gives excellent heat protection to the bundle core in communication cables. Tests show that at a given temperature, more time is required for heat-transfer through a foamed barrier than through barriers currently employed in the art. It has also been found that, when extruding the outer polymer jacket onto the metal shield, a higher temperature is temporarily retained at the metal shield-polymer jacket interface due to the insulating property of the thermal barrier located on the inner side of the metal shield. The retention of such temperatures at the interface permits extrusion of the outer polymer jacket at lower temperatures where the heat of extrusion is necessary for example, to promote the adhesion of said outer polymer jacket to an adhesivecoated metal shield.

The following examples are illustrative of the present invention and are not intended to limit the scope thereof.

Example 1 A coating composition Was prepared by admixing (1) 138 g. of a Suspension containing 40 percent solids in water of a copolymer of about 95 percent methyl methacrylate and about percent acrylonitrile said polymer containing about 35 percent by weight based on the weight of the polymer of neopentane, (2) 83.8 g. of a composition containing about 53 pencent styrene, about 43 percent butadiene, about 3 percent fumaric acid and about 1 percent acrylic acid, (3) 0.7 g. of a 30.8 percent solution of sodium lauryl sulfate and (4) 3.48 g. of a 5 percent solution of sodium polyacrylate. The resulting composition is one which, on drying of a thin layer on a solid substrate, deposits a continuous coating, which coating, on heating, expands to a cellular structure.

A 2 mil layer of the above foamable coating composition is placed on an 8 mil thick aluminum tape leaving an uncoated portion along one edge of the tape, said tape being previously coated on both sides with a thin continuous layer of a copolymer of about 92 percent ethylene and about 8 percent acrylic acid. The coated aluminum tape is longitudinally folded around the cable core or bundle so that the metal tape edges overlap at the uncoated edge without any foamable composition in the area of overlap. A polyethylene jacket is extruded over the aluminum tape. The heat of extrusion is sufiicient to expand the foamable coating on the inner side of the aluminum tape and also cause the aluminum shield to become adhesively bonded to the outer polyethylene jacket.

In the above example, a copper tape is employed in place of the aluminum tape with substantially the same results.

Example 2 The procedure of Example 1 is substantially repeated except that the aluminum tape coated with the foamable coating composition is subjected to a temperature of about 145 C. to foam the coating composition prior to construction of the cable. The resulting tape is longitudinally folded around the bundle or cable core with the foam positioned between the core and the aluminum shield. A layer of polyethylene is extruded over the aluminum shield to form the outer jacket.

Example 3 The procedure of Examples 1 and 2 are substantially repeated except that a copolymer containing 70 percent methyl methacrylate and 30 percent methyl acrylate is substituted for the 95 percent methyl methacrylate/5 percent acrylonitrile copolymer.

4 Example 4 Several tests were conducted on thermal barriers now being employed in cable fabrication processes to compare their effectiveness with that of the present invention. The tests were conducted on samples placed in position C in accordance with the assembly as shown in FIGURE 5.

FIGURE 5 is an exploded sectional View of parts assembled for the purposes of this test. It will be understood that, in practice, the parts are pressed together, the adjacent surfaces being in substantial contact with each other.

In FIGURE 5, A is a small piece of fire brick in the surface of which is imbedded the tip of a thermocouple B which is connected to a chart recorder (not shown). C represents the thermal barrier to be tested and D represents an aluminum shield (8 [mils thick, of a kind used in cable construction) coated on one side with a 2 mil layer E of an adhesive copolymer of 92 percent ethylene and 8 percent acrylic acid. F is a mil layer of polyethylene containing 2.5 percent by weight carbon black representing an outer jacket. G is a 2 mil layer of an adhesive copolymer of 92 percent ethylene and 8 percent acrylic acid which bonds a supporting piece of 50 mil aluminum metal H to the inner polyethylene jacket. I is a /2-inch layer of aluminum serving as a heat sink.

The test method was carried out by heating an assembly of layers F, G and H in an oven for at least 30 minutes to a temperature of about 480 F. The resulting laminate was removed from the oven and placed on the heat sink I, said heat sink being at room temperature. Layers C and D with E having previously been applied to D were immediately placed over the hot polyethylene layer F and immediately thereafter, A (containing thermocouple B) was placed over the thermal barrier, C, to be tested. The object of this test was to determine the maximum temperature that would be attained at the innermost surface of the foam in a cable, i.e. at B of FIGURE 5. An additional test was performed to determine the time required for heat to travel from layer F to thermocouple B and attain a temperature of about 180 F. at B. The longer the time required to reach 180 F., the better the thermal insulating property of the material. In the case of the foamable composition of the present invention, the heat from layer F caused the coating composition to foam. Table I shows the various materials that were tested, the maximum temperature attained at the thermocouple and also the length of time required to reach 180 F. through the thermally insulating material. The plastic temperature is the temperature of the polyethylene layer F immediately after the assembly of F, G and H has been removed from the heating oven. Each of the materials to be tested was placed in the position of layer C of FIGURE 5.

Tests 1 through 3 are for purpose of comparison and are not exemplary of the invention; only tests 4 and 5 are examples of the invention.

TABLE I Plastic Maximum Time to Test Material Temperature, Temperature Reach 180 F.

F. at point B, at point B,

F. seconds 1 7 mil poly (ethylene terephthalate) 480 278 4 2 9 mil polypropylene and 0.5 mil poly- 480 268 6 (ethylene terephthalate) 3 15 mil butyl rubber and 1 mil poly- 480 254 7 (ethylene terephthalate) 4 2 mil foamable coating composition 480 206 20 of Example 1.*

5 2 mil foamable coating composition 480 222 23 of Example 1 on 1.5 mil of poly- (ethylene terephtbalate) of Figure 5.

From Table I it can be seen that the composition of the present invention, as shown from the results of Tests 4 and 5, offers excellent thermal insulation. The foamed compositions deter the penetration of heat for about 13 to 16 seconds longer than one of the better thermal barriers (Test 3) now employed in the art. Moreover, the maximum temperature reached at what would be the core of the cable is considerably lower in Tests 4 and 5 than in Tests 1 through 3.

What is claimed is:

1. An electrical cable having a core of at least one insulated metallic conductor, a foamed polymer composition surrounding the core the inner surface of said foamed polymer composition having an essentially annular crosssection, a continuous, annular supporting sheathing member surrounding and adhered to the layer of foamed insulating material, and an outer polymer jacket surrounding the inner supporting member.

2. The cable according to claim 1 wherein the foamed polymer composition comprises a copolymer of methyl methacrylate and acrylonitrile, said composition containing neopentane.

3. The cable according to claim 1 wherein the foamed polymer composition is a copolymer of methyl methacrylate and methyl acrylate, said composition containing neopentane.

4. The cable according to claim 1 wherein the continuous, annular supporting sheathing member is of aluminum.

5. The cable according to claim 1 wherein the continuous, annular supporting sheathing member is of copper.

6. The cable according to claim 1 wherein the continuous, "annular supporting sheathing member is of plastic.

7. The cable according to claim 1 wherein the continuous, annular supporting sheathing member is adhesively bonded to both the foamed polymer layer and to the outer polymer jacket.

8. The cable according to claim 1 wherein the continuous, annular supporting sheathing member is adhesively bonded to the outer polymer jacket.

9. The cable according to claim 1 wherein the outer plastic jacket is of polyethylene.

10. An electrical cable having a core of at least one insulated metallic conductor, a layer of a foamed polymer composition surrounding the core, said polymer composition consisting essentially of (1) a copolymer of methyl methacrylate and acrylonitrile and (2) a binder, a supporting sheathing member of a longitudinally folded aluminum shield surrounding and adhesively bonded to the layer of foamed composition, and an outer polyethylene jacket surrounding and adhesively bonded to the aluminum shield.

11. An electrical cable having a core of at least one insulated metallic conductor, a layer of a foamed polymer composition surrounding the core, said polymer composition consisting essentially of v (1) a copolymer of methyl methacrylate and methyl acrylate and (2) a binder, a supporting sheathing member of a longitudinally folded aluminum shield surrounding and adhesively bonded to the layer of foamed composition and an outer polyethylene jacket surrounding and adhesively bonded to the aluminum shield.

12. An electrical cable having a core of at least one insulated metallic conductor, a layer of a foamed polymer composition consisting essentially of a copolymer of methyl methacrylate and acrylonitrile containing neopentane surrounding the core, the inner surface of said layer of foam polymer having an essentially annular cross-section, a supporting sheathing member of a thin layer of plastic material surrounding and bonded to the layer of foamed composition, and an outer polyethylene jacket surrounding the layer of plastic material.

13. A laminate for use as a thermal barrier and sheathing member in electric cables which comprises a cable sheathing member having a first and second side, said member having adhered to at least the first side a layer of a foamable coating composition.

14. A laminate for use as a thermal barrier and sheathing member in electric cables which comprises a cable sheathing member having a first and second side, said member having adhered to at least the first a layer of an adhesive copolymer comprising a copolymer of ethylene and an ethylenically unsaturated carboxylic acid, and a layer of a foamable coating composition adhered to the sheathing member through the layer of adhesive copolymer.

15. The laminate according to claim 13 wherein the sheathing member is of metal.

16. The laminate according to claim 15 wherein the metalis aluminum.

17. The laminate according to claim 15 wherein the metal is copper.

18. The laminate according to claim 13 wherein the sheathing member is of plastic.

References Cited UNITED STATES PATENTS 2,186,793 1/ 1940 Wodtke 174-27 2,744,291 4/ 1952 Stastny et al 26453 2,862,834 12/1958 Hiler 11741 3,206,541 9/1965 Jachimowicz 174-405 3,244,799 4/1966 Roberts 174107 OTHER REFERENCES Hyprath: German application No. 1,147,281, Pub. 4/ 1 8/ 1963.

LEWIS H. MYERS, Primary Examiner. H. HUBERFELD, Assistant Examiner. 

1. AN ELECTRICAL CABLE HAVING A CORE OF AT LEAST ONE INSULATED METALLIC CONDUCTOR, FOAMED POLYMER COMPOSITION SURROUNDING THE CORE THE INNER SURFACE OF SAID FOAMED POLYMER COMPOSITION HAVING AN ESSENTIALLY ANNULAR CROSSSECTION, A CONTINUOUS, ANNULAR SUPPORTING SHREATHING MEMBER SURROUNDING AND ADHERED TO THE LAYER OF FOAMED INSULATING MATERIAL, AND AN OUTER POLYMER JACKET SURROUNDING THE INNER SUPPORTING MEMBER.
 13. A LAMINATE FOR USE AS A THERMAL BARRIER AND SHEATHING MEMBER IN ELECTRIC CABLES WHICH COMPRISES A CABLE SHEATHING MEMBER HAVING A FIRST AND SECOND SIDE, SAID MEMBER HAVING ADHERED TO AT LEAST THE FIRST SIDE OF A LAYER OF A FOAMABLE COATING COMPOSITION. 